1. Field of the Invention
The invention relates to a drilling tool which extends in the longitudinal direction and has a drill point and a first and a second main cutting edge which extend outward. The invention also relates to a method for drilling using such a drilling tool.
2. Description of the Prior Art
Such a drilling tool can be seen, for example, from EP 0 991 489 B1. The drilling tool described therein is a twist drill for dry drilling. During dry drilling, the problem occurs that, on account of a lack of cooling, the drill expands in the cutting head region due to the generation of heat, a factor which leads to high loads occurring at the guide chamfers when the drill is withdrawn from the drill hole.
In order to avoid these high loads, an asymmetrical configuration of a chisel edge is provided in the twist drill according to EP 0 991 489 B1. On account of the asymmetrical configuration, the drill cuts a drill hole which has an enlarged hole diameter compared with the outside diameter of the twist drill. Despite the expansion in the region of the drill cutting edges which is caused by heat, the drill can therefore be taken out of the drill hole again without any problems.
For a specific field of application, namely the drilling of holes in printed circuit boards formed from synthetic resin, a special drill can be seen from DE 26 55 452 A. Said special drill also has an asymmetrical configuration of the drill head in the region of the cutting edges in order to enlarge the drill hole. This is optionally achieved by the cutting edges being designed with different lengths or by said cutting edges being arranged at different point angles. The particular problem which arises during the drilling of printed circuit boards, namely that drill dust is produced which can be pressed against the wall of the drill hole, is thereby taken into account. Due to the asymmetrical configuration, a drill hole larger than the outside diameter of the drill is produced, such that the drill is at a distance from the wall of the drill hole on one longitudinal side.
In contrast thereto, however, in the normal case highly symmetrical configurations are desired in drills, such that concentric running of the highest possible precision and high-precision drilling are achieved. Here, asymmetry may perhaps occur only as grinding errors. In contrast, the drills described above use the asymmetry specifically to avoid problems in special applications. With the asymmetrical configuration, however, asymmetrical loading and thus increased wear of the drilling tool also occur, as a result of which the service life is reduced. This applies in particular to drills such as carbide drills or drills having cutting inserts made of a special cutting material which are intended for machining high-strength materials, for example steels.
On account of the asymmetrical configuration, the drill is particularly pressed on one side against the wall of the drill hole during the drilling operation. A guide chamfer is normally provided circumferentially on the “land”. The drill bears with this guide chamfer against the wall of the drill hole. In conventional drilling tools having a highly symmetrical configuration, the two guide chamfers in a double-edged drill bear diametrically opposite one another in each case against the wall of the drill hole and are uniformly loaded. Due to the asymmetrical configuration, one of the two guide chamfers is now loaded to an excessive degree, such that the wear thereof is markedly higher. In addition, the risk of parts of the guide chamfer chipping in the event of caking of chip parts on the wall of the drill hole is greater.
The object of the invention is to specify a drilling tool which has asymmetrical configurations and high wear resistance.